With the development of the IC technology and the continuous scaling down of the critical dimension, new materials and processes are introduced in the IC manufacturing process to satisfy the overall functionality of the IC products. Therefore, alignment accuracy between different structures in a semiconductor device has become more important. As the critical dimension has reduced below 65 nm, tiny alignment shifts between the contacts and the gate structures, for example due to mask misalignment, may cause the contacts short the gate structures to the substrate and lead to electric leakages, and may further result in performance degradation or even failure in the semiconductor device.
As shown in FIG. 1, which illustrates a schematic of a 6T SRAM structure through Transmission Electron Microscopy (TEM), the contact hole 101 formed between the poly silicon gate 102, 103 offsets and makes the spacer of the gate 102 thinner, such alignment shift may cause the failure of the whole SRAM structure.
Therefore, it is of great importance to detect and measure the alignment shift of the contact holes relative to the gate structures in a semiconductor device. Currently, the detection is mainly carried out through optical methods. Referring to FIG. 2, which is a flow chart of a conventional method of detecting the alignment shift of the contact holes relative to the gate structures, the method comprises:
Step S1, forming gates on the active areas of a semiconductor substrate;
Step S2, forming multiple evenly spaced contact holes on the gates as well as the other active areas;
Step S3, detecting and calculating the alignment shift between the contact holes and the gates through an optical method; wherein the gate length is determined according to the diameter of the contact hole, the spacing between the adjacent contact holes, and the gate width.
However, with the continuous shrinking of the device dimension, the alignment shifts may not be accurate detected through optical detecting methods due to the resolution limitation thereof, and thus the requirements of precise process control may not be satisfied. Consequently, it is necessary to provide a new method of detecting and measuring the alignment shift of the contact holes relative to the gate structures in a semiconductor device.